Steel / Fully Rigid SS Build

okay so... I'm still relatively new to mountain biking. I rode bmx for 10 years and have commuted on my SS CCX bike for 2 now. I definitely have the mechanical aptitude to put a bike together, but I don't really know where to look for the parts, or, more importantly, what parts to look at.

Can someone point me in the right direction?

I want a steel, 26 or 27.5, fully rigid SS with mechanical discs and tubeless tires. Feel free to suggest any parts at all.